Can we do exercise before yoga?

It is good to exercise before practicing yoga.
It is always best to have a cold water shower before starting Yoga, specially after your exercise routine.
It is also advised that you have an empty stomach condition. Depending on the Asana that you practice, this should range from 0-4 hrs.
If you meet these three conditions, it is very likely that you will have deeper experiences once your eyes are closed and you are in the Yogasana.

Source :Sadhguru Jaggi Vasudev of Isha Foundation
I am a 19-year-old male. My chest size is 38, waist 36 bicep 12, weight 73 kg and BMI 23. What would I have to do to get to a 32 waist?

70–80% percent is about what you eat. Eat healthy foods, like vegetables, eggs, meat, fruits, coconut oil. These foods are high in nutritional value and low in calories.And cut down on process food, junk food, fried food, sweets, liquid calories (alcohol, juices, sodas, diet sodas will make you hungry), pizza,

If I walk four miles, 5-6 times a week, and eat 1200 calories a day, should I expect to lose weight?

You will lose some weight. But in order to do that you will need to fast peace your walk or jog. I recommend to incorporate some strength muscle exercises and flexibility. With these you will not only start losing weight, but your muscles will